The Edelbrock #608115 Performer RPM cylinder heads are designed for 1958-1965 Chevrolet “W” Series 348/409 big-block applications. The Edelbrock #608115 Performer RPM heads are perfect for street performance, daily drivers, street rods and muscle cars where seat-of-the-pants performance is needed. These cylinder heads deliver great throttle response and power from 1500 to 6500 rpm. They also feature the highest quality intake and exhaust valves and valve springs. For use on pre-pollution controlled vehicles only.INSTALLATION NOTE: • EQUIPPED WITH SPRINGS FOR USE IN MECHANICAL FLAT TAPPET AND HYDRAULIC ROLLER CAMSHAFT APPLICATIONS • Original port locations and valve angles for bolt-on convenience • CNC port matched 220cc intake and 115cc exhaust ports for optimum flow and power • Improved port design for more performance • Hardened spring cups, screw-in rocker studs and hardened guideplates • 1958 348 c.i.d. applications will require modifications to cylinder head (contact Edelbrock tech department for detailed information) • Use Edelbrock head gasket sets. • May be used on 348 engines with valve lifts up to .550" only. • Recommended spark plugs are Champion RC12YC or equivalent 14mm x 3/4" reach with flat gasket seat. • These heads will only fit 1958-65 348 c.i.d. "W" engines if an Edelbrock #5409, #7159 or stock large port 409 c.i.d. manifold is used. • Proper cylinder wall notching must be checked to ensure exhaust valve to cylinder clearance. • Deck thickness: 9/16" • Rocker stud diameter: 7/16" • Pushrod diameter: 3/8" (width of slot in guide plate).